Hard to say.Update. I put the chicks in the new addition while I was painting the outside. I placed a barrier at the open junction so they can see out and poke their heads out if they wish. They have been staying in there since 11 am. Four of the hens came in to lay eggs and then left dining their egg songs. The crate is right next to the coop door with the crate door open. It is pretty much ignored but all of the hens and chicks are used to it. Last night was the first night with the roofing addition open ended and no one slept in there. The hens went to their old spaces. And the chicks were inside the crate inside.
I just moved the addition barrier to make it wide enough so they can leave if they want to. They have food and water in there with them. To get out they will need to walk through the original space or wait for us to open the door. I would rather they choose to go out on their own. Do you think they will be able to find their way back if they do?
You'll probably have to juggle things for at least a few days.
I'd leave them confined to the addition tonite,
then open the barrier in the morning after the existing birds leave the coop,
and see what they do.